A Valence Bond Model for Electron-Rich Hypervalent Species: Application to SFn (n=1, 2, 4), PF5, and ClF3

Some typical hypervalent molecules, SF4, PF5, and ClF3, as well as precursors SF (4Σ− state) and SF2 (3B1 state), are studied by means of the breathing‐orbital valence bond (BOVB) method, chosen for its capability of combining compactness with accuracy of energetics. A unique feature of this study is that for the first time, the method used to gain insight into the bonding modes is the same as that used to calculate the bonding energies, so as to guarantee that the qualitative picture obtained captures the essential physics of the bonding system. The 4Σ− state of SF is shown to be bonded by a three‐electron σ bond assisted by strong π back‐donation of dynamic nature. The linear 3B1 state of SF2, as well as the ground states of SF4, PF5 and ClF3, are described in terms of four VB structures that all have significant weights in the range 0.17–0.31, with exceptionally large resonance energies arising from their mixing. It is concluded that the bonding mode of these hypervalent species and isoelectronic ones complies with Coulson’s version of the Rundle–Pimentel model, but assisted by charge‐shift bonding. The conditions for hypervalence to occur are stated. Charge‐shift bonding is the glue that makes electron‐rich hypervalent molecules stable in the framework of the valence bond (VB) version of the Rundle–Pimentel model (see figure). This model emerges directly from high‐level VB calculations, thus ensuring the correctness of the interpretations. It provides a unified description for all studied electron‐rich hypervalent species, and allows both extra insight and improved predictive power compared with other traditional or recently proposed models for hypervalence.